The present invention relates to a method for preparing a multi-layer coating having anti-reflective properties on the outer surface of an image display device, such as a cathode ray tube (CRT), a liquid crystal display (LCD), etc.
Transparent panels for the display screens of image display devices, such as a CRT or LCD, are made of glasses or plastics, which strongly reflect external light, making it difficult to view images formed thereon. Especially, display devices constituted with various CRTs are used in terminals of computer systems so that increased importance should be placed on such anti-reflective properties.
In order to prevent the surface of the display panels from reflecting external light, the glassy surface of the panel is etched with chemical reagents, such as silicofluoric acid (H.sub.2 SiF.sub.6), or with sandblasting process to form minute irregularities, thereby imparting a non-glare property. However, these methods are disadvantageous in that such a chemical treatment creates minute irregularities which are weakened and thus subject to being crushed easily and that direct damage to the screens makes their regeneration impossible.
In recent years, an alcohol solution containing Si(OR).sub.4 has been spray-coated on the outer surface of the panel to form numerous fine irregularities. Japanese Patent Laid-open Publication No. sho 61-118932 discloses a cathode ray tube having an antistatic and non-glare coating in which an alcohol solution of Si(OR).sub.4 is spray-coated on the outer surface of a panel and heated at 150.degree. C. or less, to form a SiO.sub.2 film with silanol groups. Since the heating temperature is relatively low, some silanol groups remain in the siloxane structure, and the hygroscopic nature of the -OH group endows the film with antistatic properties. The method is advantageous in that the coating is easily prepared and regeneration is possible, but has the problem of low resolution.
Japanese Patent Laid-open Publication No. sho 64-76001 discloses a non-glare coating which is prepared by spray-coating an alcohol solution of Si(OR).sub.4 containing MgF.sub.2 particles of an average particle size of 0.01-1 .mu.m dispersed therein on the surface of the glass, and heating the resulting coat to form a SiO.sub.2 film which fixes fine MgF.sub.2 particles and forms numerous projections on the surface of the panel.
U.S. Pat. No. 4,949,282 discloses a process for producing image display panels, which comprises the steps of dispersing fine SiO.sub.2 particles of 100-10,000 .ANG. in diameter in an alcoholic solution of Si(OR).sub.4 and dispersing particles of electroconductive metal oxides and/or hygroscopic metal salts, applying the resulting suspension onto the panel surface, heating the resulting coat to decompose the Si(OR).sub.4, and forming a thin SiO.sub.2 film to coat fine SiO.sub.2 particles and the additive particles in order to fix them on the surface of the panel. FIG. 1A shows an enlarged sectional view of this antistatic coat having non-glare function formed on the outer surface of the panel (1). The SiO.sub.2 film (2) has a non-glare function due to the SiO.sub.2 particles (3) as well as an antistatic function. However, according to the method of spraying an alcoholic suspension of Si(OR).sub.4 containing fine particles and fixing them on the outer surface of the panel, the diffusion effect due to the fine particles decreases the degree of resolution.
Generally, anti-reflective coating requires more than two layers, and typically four layers, in order to get a wider range of wavelengths having low reflectance. According to the method of spraying an alcoholic solution of Si(OR).sub.4, the sprayed liquid particles are deposited more thickly when approaching toward the edge of the glass panel face, so a uniform layer throughout the glass surface is difficult to obtain. Such a problem of uneven coating has become serious as the number of coated layers increases, so multiple layers are hard to produce by the spraying method.
Recently, a vacuum deposition, sputtering, or CVD (chemical vapor deposition) method is also used to form multiple layers of conductive metals on the outer surface of the panel so that the antistatic and anti-reflective properties are provided. FIG.1B is an enlarged sectional view of an example of multiple deposition layers which is composed of MgF.sub.2 layers (4a,4b) and TiO.sub.2 +Pr.sub.6 O.sub.11 layers (5a,5b). These layers are formed by alternately depositing MgF.sub.2 of a given refractive index (n.sub.1 =1.38) and TiO.sub.2 +Pr.sub.6 O.sub.11 of a given refractive index (n.sub.2 =2.06). Such multiple deposition layers have an advantage of low reflectance (below 0.1), but these methods involve significant problems. That is, these methods require a large-scale apparatus, and further, many operation steps and high production costs are required in order to produce a large-scale display tube.